Interest has been building to find an effective treatment for migraines and to address their underlying cause in persistent migraine sufferers. Research is growing to link migraines with visual discomfort, often associated with perceptual distortions, particularly in printed text. This is commonly referred to as visual stress, photophobia or Meares-Irlen syndrome. These perceptual distortions typically occur where the text appears to move and blur, resulting in eye-strain, headaches and reading difficulties. The condition is estimated to be present in about 40% of poor readers and in at least 5% of the general population in varying degrees.
It has been shown that the use of eyewear containing ‘precision tinted lenses’ can prevent, or at least reduce, symptoms of visual stress experienced by a patient, making it easier for the patient to read without discomfort. Some migraine sufferers report that they can prevent or curtail their aura by wearing their precision tints.
The Intuitive Colorimeter®, manufactured and distributed by Cerium Visual Technologies Limited (Company No. 02817206) is an example of a visual stress assessment device used for the precision selection of tinted lenses and is now in widespread optometric use in the UK and overseas. The Mark 1 version of the device is described in GB-B-2246427, whilst the Mark 2 and 3 versions are described in EP-B1-0803679. The basic principle of its use is as follows; the device illuminates a page of text with coloured light of a given spectral power distribution (loosely referred to as ‘tint’) which is observed by a patient. The spectral power distribution refers to the distribution of light energy or power across the visible spectrum. An optometrist varies the tint of the coloured light by adjusting its hue and saturation independently until a tint has been selected that most reduces any visual discomfort experienced by the patient. Bespoke software is then used to select an appropriately dyed lens to match the selected tint which reduces perceptual distortions and increases reading fluency.
A detailed discussion of the Intuitive Colorimeter® (Mark 2) and its merits is provided in Wilkins, A. J. and Sihra, N. (2001), A colorizer for use in determining an optimal ophthalmic tint. Color Res. Appl., 26: 246-253. In summary, a mechanical system is used for colour mixture in which seven coloured filters are equally disposed around the circumference of a cylinder. Light from a fluorescent lamp within the cylinder passes through two neighbouring filters via a square aperture into a viewing chamber where it is mixed by multiple reflection. The proportion of light transmitted through the filters varies with the rotation of the cylinder, continuously changing the hue. The cylinder can slide along its axis bringing a third (neutral grey) filter into the aperture, reducing the saturation of the colour. Attenuation filters can also be inserted or removed so as to adjust the luminance (i.e. brightness) of the light reflected from within the chamber.
Part of the ingenuity of the Intuitive Colorimeter® is its ability to enable independent variation in hue, saturation and luminance such that any tint can be produced within a wide gamut. The variation in the colour or ‘tint’ produced by the device is continuous, rather than necessarily occurring in discrete steps, enabling the best possible tint to be selected. The spectral power distribution of the light produced in the instrument also substantially matches that obtained from tinted spectacle lenses worn under conventional fluorescent lighting. This feature is nontrivial considering that coloured light is combined in an additive manner within the device, whereas it is combined in a subtractive manner in dyed spectacle lenses.
AU-A1-2006236057 discusses a number of limitations which are identified in the Intuitive Colorimeter®; namely its size, its mechanical nature and the need for a skilled operator to be present to control it. An alternative method and apparatus is suggested that attempts to address these limitations by providing an electronic display device wherein the step of selecting a tint (including modifying the saturation, luminance and hue independently) is computer implemented. The display device utilised is a liquid crystal display (LCD), connected to a standard computing system. The methods by which the hue and saturation are manipulated are not described, and are critical, partly because the chromaticity of the screen needs calibration if the hue and saturation are indeed to be manipulated separately at a constant luminance. It is not possible to achieve a spectral power distribution from an LCD (which inherently comprises red, green and blue pixels only) such that it matches the spectral power distribution obtained when coloured filters are worn under conventional light sources, at least those filters that use conventional dyes.
There is therefore a need for an improved device that addresses the abovementioned limitations in the prior art.